Superficial doses from serial tomotherapy delivery

Serial tomotherapy patients are treated using a commercial computer-controlled treatment planning and delivery system and a linear accelerator in arc mode. Target volumes are irradiated using sequential 1.68-cm-thick slices with the patient moved between each successive slice delivery. Due to the method of delivery, superficial doses from a course of intensity modulated radiation therapy may be significantly different from doses delivered with conventional radiation therapy. An experimental investigation was conducted to determine the superficial depth-dose distribution for geometric target volumes. Treatment plans were conducted for 5-cm-diam, 5-cm-long right cylindrical target volumes embedded in a 16-cm-diam, 12-cm-long water-equivalent cylindrical phantom. Three experiments were conducted with centrally located, off-centered, and superficial targets. Doses at superficial depths were measured using 3 x 3 x 1 mm3 thermoluminescent dosimetry chips. For all three targets, the measured doses were slightly greater than the calculated doses. Doses measured in the buildup region for the superficial target indicated that bolus would be required to provide the prescribed dose at the phantom surface.